Conventionally, a header tank of a heat exchanger such as a radiator is configured by integrating a metal core plate to which each tube is joined, and a resin tank main body part that defines an inner-tank space. A packing (sealing member) made of an elastic member such as rubber is disposed between the core plate and the tank main body part, and the core plate and the tank main body part are sealed by compression of this packing at the core plate and the tank main body part.
In a heat exchanger described in Patent Document 1, a core plate includes a tube joint surface where a tube is joined, and a groove part formed at an outer peripheral edge part of the tube joint surface. An end portion of a tank main body part on the core-plate side is inserted in the groove part of the core plate. The tank main body part is fixed to the core plate with a packing clamped between the groove part of the core plate and the end portion of the tank main body part.